Without limiting the scope of the invention, its background is described in connection with the plasma etching of silicon wafer surfaces, as an example.
As MOS technology advances, increasingly thinner gate oxide layers are created, requiring greater control over the etching process. With thinner layers comes an increased demand for improved efficiency, decreased failure rates, and greater control over the process. One area that would greatly benefit from increased control of the processing of surfaces is the plasma etching process.
Heretofore, in this field, plasma etching has been accomplished with only generalized control over the ion energy flux and plasma potential. Using conventional inductively coupled plasma technology, a plasma is generated using antennas, or coils, that create radio-frequency (rf) electromagnetic fields within a vessel containing gas. To eliminate capacitive coupling of the coil to the plasma a Faraday shield is sometimes used, thereby eliminating most of the rf fluctuation of the plasma potential.
In present technology, most of the plasma reaction vessel or chamber is composed of metal tied to ground. These reactors hold the plasma potential of discharges formed within them at a value near ground, with some rf variation in plasma potential depending on the type of discharge source and the presence or absence of a Faraday shield. The bulk of the plasma is free of electric fields, except near the walls, where a sheath develops which contains the more mobile electrons. This sheath attracts positive ions to the surfaces in the chamber, including the surfaces to be processed.
In a silicon wafer etching process, the wafer is biased with a separate rf supply to give the surface of the wafer a dc bias potential with respect to the plasma potential to attract the ions to the wafer with greater energy. The plasma potential, however, may change during processing as the metallic walls of the chamber become deposited with etch products. Over time, plasma potential changes can shift ion energy distributions at the surface of some substrates affecting process control.
The plasma potential is established by the most positive potential in the chamber with which the plasma is in contact. As the portions of the wall that are made of dielectric material, or which have become deposited with dielectric material, are bombarded with positively charged particles, these portions of the wall charge up and change in potential. The influence of these charged surfaces on the plasma potential will depend on their surface area with respect to the grounded surfaces in the chamber and will change over time.
It is common practice to clean etching reactor chambers routinely to remove the build up of deposited material and restore the plasma both electrically and chemically to that of a new reactor. Thus, although the changes in plasma potential may stabilize after a certain time due to a complete coverage of the walls with deposited material, changes in the plasma potential will once again occur after the reactor has been cleaned and may cause considerable downtime or unproductive use of the reactor.
Therefore, a need has arisen for an apparatus that provides for the precise control of the plasma potential within a chamber and a method of controlling the plasma potential within the chamber, thereby increasing the control over the intrinsic characteristics of the plasma process.